This invention pertains to an improved coal liquefaction process with enhanced product recovery and process solvent obtained by recycle of a heavy solvent refined coal fraction from the critical solvent deashing stage.
A wide variety of processes have been proposed for the conversion of coal to a low-sulfur, low-ash solid carbonaceous fuel and distillate hydrocarbon fuel by solvent refining of the coal in the presence of a hydrogen donor solvent. Typically, upon heating and liquefaction of the coal in such a process, light gases are first separated and the remaining slurry vacuum distilled to produce a light distillate product, a recycle solvent, and a heavy fraction including, among other things, residual solvent, dissolved coal products, undissolved and heavy coal products, mineral or ash material and unconverted coal macerals.
It is well-known that further products may be recovered by solvent deashing this vacuum still bottom stream or residium. Such a process, sometimes referred to as a "critical solvent deashing" process, is disclosed, for example, in U.S. Pat. No. 4,070,268. As indicated in that patent, typical products of the solvent deashing stage include a stream comprised of soluble coal products, the composition of which is rich in preasphaltenes but which is essentially free of ash and unconverted coal particulate matter. This low-sulfur carbonaceous material is ideally suited for combustion in an environmentally acceptable manner. A bottom stream of insoluble coal products and ash is also normally produced as well as an overhead stream of distillate which is recycled as solvent in the deashing stage.
As seen in U.S. Pat. No. 4,164,466, the solvent deashing stage often comprises a number of product separation zones, typically of successively higher pressure, all maintained at relative high temperature. U.S. Pat. No. 4,164,466 also discloses a process wherein the underflow from the second separation zone is recycled to the entry mixing zone in the solvent deashing stage.
In the process disclosed in U.S. Pat. No. 4,189,372, a portion of the underflow from the third and fourth separators is hydrogenated and returned to the coal liquefaction slurry tank. Substantially all other intermediate flows from the second, third, and fourth separators are recycled to the mixing zone at the entry of the solvent deashing stage.
In U.S. Pat. No. 4,119,523, the underflow from the first separator in a solvent deashing stage is extracted, to separate resulting ash and undissolved coal, and the remaining extract returned to the coal liquefaction stage.
Notwithstanding this prior art, there remains a need for still further enhanced product recovery in a coal liquefaction process including solvent deashing.
It is therefore the general object of the present invention to provide such an improved process.